COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                       Number 2773

                                        Konkoly Observatory
                                        Budapest
                                        9 August 1985

                                        HU ISSN 0374 - 0676


           PERIOD AMBIGUITY RESOLVED FOR HD 8358


   Although UBV photometry of Fekel, Hall, and Henry (1984) succeeded in
discovering that the chromospherically active G-type star HD 8358 was variable 
with a range of 0.14mag in V, they were not able to determine the photometric 
period unambiguously because their observations had been made only
once each night at very nearly 1-day intervals. The three possible periods
were 0.520d +/- 0.001d, 1.085d +/- 0.002d, and 12.75d +/- 0.25d. With all three periods
the light curve shape was asymmetric in the same sense: rapid rise to maximum 
followed by a stillstand between maximum and minimum.
   In this paper we present V-band photometry obtained with the Barksdale
14-inch reflector on 26 nights during the last three months of 1984. The
individual DeltaV magnitudes have been sent to the I.A.U. Commission 27 Archive
for Unpublished Observations of Variable Stars (Breger 1985), where they
are available as file no. 151. Each of the 636 values of DeltaV is based on an
intercomparison between variable and comparison star and has been corrected
for differential atmospheric extinction and transformed differentially to V
of the UBV system. The comparison star was BD - 0d 212 = SAO 109848 and Delta is
in the sense variable minus comparison. Because on most nights HD 8358 was
observed continuously for a large fraction of the night (up to 7.7 hours),
this photometry succeeds in removing the ambiguity in the period: the shortest 
of the three, P = 0.52d, is correct.
   Figure 1 is a plot of all 636 individual deltaV values, phase being computed
with P = 0.520d. Here we see immediately that the light curve is
now double-humped, i.e., markedly different from the shape two years earlier,
seen in figure 1 of Fekel, Hall, and Henry (1984).
   To refine the period, we fit the light curve by least squares using a
Fourier series containing only terms in 28. The smallest sum of the squares
of the residuals occurred at P = 0.52006d +/- 0.00007d. The resulting full 
amplitude was 0.065mag. Light minima, taken at the middle of Barksdale's 
observing season, occurred at JD(hel.) 2446012.6919 and 2446012.9519. And the rms
deviation was +/- 0.033mag, which is quite large and needs to be discussed.


                             [FIGURE 1]

 Figure 1   Light curve of the chromospherically active star HD 8385, 
 where phase is computed with a 0.52-day period. The double-humped shape 
 indicates two spot groups were present in late 1984. The large scatter, 
 produced in part from intrinsic cycle-to-cycle changes, is discussed in 
 the text.


   Three factors are responsible for the large rms deviation. (1) A sin (2theta)
curve is an imperfect description of the mean light curve shape. (2) The
data were not purged or sanitized in any way prior to analysis, so a few
nights of inferior photometric quality probably were included. (3) The
light curve underwent intrinsic changes in shape during the 144 orbital
cycles which were observed; overlays of separate plots for individual
nights demonstrated that the brightness at both maxima and both minima
changed as much as 0.05mag, with no obvious trend or pattern of repetition.




   While this paper was in preparation, we received a preprint from
Bopp et al. (1985) presenting extensive photometric, spectroscopic, and
far-ultraviolet observations of HD 8358. Perhaps we can say here that
they, too, found the light curve shape to change dramatically on a variety
of time scales.
   The light curve changes are surely a consequence of starspot activity.
The double-humped shape seen in our Figure 1 indicates two spot groups
were present in late 1984. The dramatic changes in shape and the rapidity
of those changes make HD 8358 a good candidate for the study of starspot
evolution. The rapidity itself, however, will require that future monitoring 
be extensive and as nearly uninterrupted as possible, in order to
maintain proper cycle count from season to season and to define interesting 
multiple periodicities which may be present.





                       WILLIAM S. BARKSDALE, JR.
                       633 Balmoral Road
                       Winter Park, Florida 32789

                       DOUGLAS S. HALL
                       W. TIMOTHY PERSINGER
                       Dyer Observatory
                       Vanderbilt University
                       Nashville, Tennessee 37235





References:

Bopp, B. W., Ake, T. B., Goodrich, B. D., Africano, J. L., Noah, P. V.,
   Meredith, R. J., Palmer, L. H., and Quigley, R. 1985, preprint. [BIBCODE 1985ApJ...297..691B ]

Breger, M. 1985, P.A.S.P. 97, 85. [BIBCODE 1985PASP...97...85B ]

Fekel, F. C., Hall, D. S., and Henry, G. W. 1984, I.B.V.S. No. 2543.


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